1. Field of The Invention
This invention relates to an apparatus for maintaining a constant surface speed of a rotating work piece being cut by a moving tool.
2. Description of The Prior Art
When using a lathe to machine a rotating piece of metal stock, it has been found that the productivity of machining and the quality of the surface finish of the product can be optimized if it is possible to maintain a constant surface speed of the work piece being cut. The maintenance of the constant surface speed presents a problem, since, as the diameter of the work piece is reduced, the surface speed of the rotating metal stock tends to decrease if the RPM of the stock remains constant, because the surface speed is proportional to the angular velocity of rotation of the stock multiplied by the radius of the stock. Therefore, in order to continue to maintain constant surface speed of a rotating work piece, whose radial dimension is continuously being reduced, one prior art technique calls for a change in the spindle speed in response to sensed incremental changes in the diameter of the work piece. Thus, if one wants to cut 10 radial inches from a work piece, and also wants to change the surface speed every time the radius of the work piece is reduced 1/4 of an inch, then 40 separate programming control blocks would be required to initiate 40 separate commands. Using this approach, the work piece would have a very rough surface finish, and the precision of the cut of the work piece would likewise be limited. In order to obtain a more accurate, smoother surface finish and a more precise machined part, it would be necessary to provide more programming commands. However, further increasing the programming commands could ultimately reduce the response time of the system to an unacceptable level.
Alternate analog techniques for accomplishing the above purposes have been provided by manually controlling the setting of a potentiometer or by automatically controlling the gain of an analog amplifier as a function of the spindle motor speed. However, again these analoging techniques are limited both in accuracy and in response time.
A further technique has been described in U.S. Pat. No. 3,665,170. This patent essentially describes the product of the actual radius of the work piece and the spindle speed being integrated over a period of time by using a differential digital analyzer. However, since it is necessary to await the arrival of the overflow output from the differential digital analyzer over the period of the integration before corrective action can be taken, the response time of this system is accordingly slow. When the radius of the work piece becomes smaller and smaller, the RPM of the work piece, or spindle holding the work piece, must change faster in order to maintain a constant surface speed of the work piece. Since the integration technique used in this patent is inherently slow, its poor dynamic response renders its use inadequate for increased cutting ranges, especially when high cutting speeds are required.